The present invention relates to a biaxially oriented polyester film. More particularly, it relates to a biaxially oriented polyester film which, when used as a base film for high-quality magnetic recording media, can highly satisfy the running property and electromagnetic transducing performance required of the magnetic recording media, and is also excellent in chipping resistance, prevention of dropout, handling property and winding property in the film-forming and magnetic recording medium production processes.
Biaxially oriented polyester films are widely used as an industrial material because they have various advantageous properties in a well-balanced manner and high cost performance.
Recently, in use of polyester films for magnetic tapes in particular, it is noted that the property requirement for the base film is becoming more and more strict for realizing higher quality of magnetic tapes. Specifically, in order to improve electromagnetic transducing performance, it is strongly desired that the film surface be flat, and particularly it is an essential condition that there exist in the film surface no coarse projections which might become a cause of dropout. On the other hand, it is also necessary to roughen the film surface to a pertinent degree to reduce the friction coefficient of the film so as to enhance the running property of the magnetic recording medium.
On the other hand, in the film working steps, for example in the steps of magnetic layer coating, calendering and take-up of the film in the manufacture of magnetic recording media, the problem exists that the film surface could be chipped off through contact with the rolls or guides, and the resulting flaw deteriorates the electromagnetic transducing performance or the chipping dust causes dropout.
Also, with prevalence of video software in recent years, high-speed dubbing devices have come to be used for enhancing soft video tape productivity, and in use of such high-speed dubbing devices, high quality of the magnetic tape, and of the base film for such magnetic tape, has become an essential requirement. Since the magnetic tape running speed at dubbing is faster than the tape run in normal video replay, there is required a design different from that required for improvement of running property of the conventional magnetic tapes. For example, it is required that the magnetic tape has high running stability and won't move zigzag in high-speed run, and that the tape remains safe from being chipped off during running so that there will be no risk of dropout being caused by chipping dust. That is, the magnetic tape is required to have high-degree chipping resistance. Further, in addition to such chipping resistance in high-speed run, it is also required for the magnetic tape to show high durability, namely extraordinarily high degree of wear resistance, in repeated use of the magnetic recording medium.
Beside these strict quality requirements, request for cost reduction is also intense, and it is essential that the base film productivity is high.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that by containing the inorganic particles having a specific average diameter as well as the crosslinked polymer particles having a specific average diameter in a biaxially oriented polyester film, the above problems can be solved.
The present invention has been attained on the basis of the above finding.